<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        /*body{
            font: 12px "simhei",Arial,Helvetica,sans-serif;
            color: #666666;
        }*/
        body>div{
            width: 300px;
            background-color: rgba(0,0,0,0.3);
            padding: 10px;/*四个方向的内边距   实现缩进效果*/
            position: relative;/*相对定位*/
        }
        input{
            width: 260px;/*300-40内边距*/
            border: 0;
            padding: 10px 20px;
        }
        img{
            position: absolute;
            right: 40px;
            top: 14px;
        }
        p{
            font-size: 12px;
            color: red;
            margin: 0;
        }

    </style>

</head>
<body>
<div>

  <input type="text" placeholder="请输入用户名">
  <img src="http://doc.canglaoshi.org/tstore_v1/images/login/yhm.png" alt="">
  <p>用户名不能为空!</p>
</div>

</body>
</html>